To measure for converter bolt pattern, just measure from the crank centerline to the bolt hole centerline and multiply by two. This will establish your converter bolt pattern. This measures roughly 5-3/8 inch, times two, would be the small, 10.75-inch converter pattern for a TH350 or Powerglide. (Image/Jeff Smith)

I have a 5.7 LT1 from a ‘94 Trans Am I want to mate it to a ‘79 TH400 transmission. What flexplate do I need? This has been a very frustrating experience so I hope you can help me. Thanks

B.M.

There are multiple flexplate/flywheel bolt patterns for the small block Chevy—especially if you include the LS engine family. We’ll concentrate on your situation.

Early Gen. 1 Chevy small blocks, defined as engines from 1955 through 1985, employed a two-piece rear main seal. All these engines use the standard six-bolt pattern that is interchangeable with two-piece rear main seal big-block Chevy engines. Be careful with that however, since it’s not quite that simple with regard to external balance (we’ll get to that in a moment).

In 1986, Chevy converted to a one-piece rear main seal which altered the crankshaft bolt pattern to make sure that you could not accidentally use the wrong flexplate or flywheel. The main reason for this is the older two-piece rear main seal engines employed a small offset weight on the side of the crankshaft mounting flange. It’s not possible to add this offset weight to the one-piece rear main seal flange, so instead the engineers relocated this weight to the flexplate or flywheel. The engine is still considered an internally balanced engine so there is no offset weight on the harmonic balancer, but there is an offset weight on the flexplate.

Compounding the potential confusion is that the 1970-era 400 cubic inch small block Chevy was an externally balanced engine, so there is a weight on those flexplates but it’s not the same amount of external weight as the one-piece rear main seal engines and the crankshaft mounting bolt pattern for the 400 engines is the same as the two-piece rear main seal small-blocks.

Plus, there’s the external weight used on externally-balanced 454 engines. The 454 external weight is greater than the external balance weight on a 400 small block, so each of these two engines call for a different flexplate than the internally balanced small block Chevy.

Confused yet? It gets more complicated so hang in there.

Now that we have a handle on the crankshaft mounting flange and external weights, next we need to address two more variables. The early small block two-piece engines used a 153-tooth flexplate/flywheel while the higher horsepower engines moved to a larger diameter, 168-tooth flexplate/flywheel.

Torque converter patterns can add complexity to the selection of a new flexplate since that now adds another set of variables. The small pattern measures 9-3/8 inches, while the large pattern is 10 inches across. Several aftermarket companies sell flexplates that can accommodate this conversion. First of all, the flexplate must match the one-piece rear main seal bolt pattern which also means it will include an offset weight. Be careful when looking up these aftermarket flexplates, because some may call out an internally balanced engine where the exterior weight is not included.

We referenced the TCI catalog and found a flexplate for a one-piece rear main seal with an external balance weight that accommodates both the small and large torque converter bolt pattern. The only issue is that this is a 168-tooth flex plate and not a 153 tooth that it appears you need. The flexplate will fit inside the TH400 bellhousing but this will require using a different starter motor since the starter on those stock engines were designed for a 153 tooth flex plate. The TCI flex plate part number is TCI-399773.

One way to tell if your starter will fit a large diameter flexplate is to look for an offset bolt pattern on the nose of the starter. Starters intended for the smaller 153-tooth flexplate/flywheel will have a straight across bolt pattern.

As a tech tip for installation, always make sure you install the flexplate with the raised converter bolt pattern pads positioned toward the torque converter. It’s really easy to overlook this and install the flexplate backwards so be sure to make a note of this when installing the flexplate. If the raised portion on the flexplate is not facing the torque converter, it will not bolt up properly to the converter. You don’t want to discover this with the transmission already in place. These details should help you find the correct flexplate.

Torque ConverterPattern
PG/TH350 (Small)10.75"
TH400 (Larger)11.50"
LS (Early 280mm)11.07"
LS (Late 300mm)
4L80E		11.50"
How to measure torque converter pattern:

Measure from the crankshaft centerline to the centerline of one of the converter mounting holes on the flexplate and multiply by 2. As an example, if the distance from the crank centerline is 5.75 x 2 = 11.50 inches.
Keywords
Flexplate, LT1, TH400, transmission swap
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.